Conventionally, materials whose surface roughness is managed, such as a tin plate, have faced a problem of a partial abnormality in surface roughness caused by abrasion of rolling mill rolls or a difference in the amount of drawing of rolling oil. To detect this partial abnormality, there are needs for measuring surface roughness of a material being managed as a measurement object. As a technique for measuring surface roughness in a noncontact manner, angle-resolved scattering using the degree of scattering of light reflected on a surface (JIS B0681-6 (2014)) is known, and some surface roughness measurement methods applying this principle have been proposed.
For example, Patent Literature 1 discloses a laser type surface roughness measurement apparatus that measures surface roughness of a measurement object according to spread of a diffraction pattern of laser spot light. In addition, Patent Literature 2 discloses a method of projecting a reference pattern on the surface of a rough-surface object, forming an image of fluctuation of a reflected image of the reference pattern, and measuring surface texture of the rough-surface object on the basis of luminance distribution of the fluctuation and magnitude of the fluctuation of the reflected image, which are obtained by analyzing a captured image obtained by imaging the fluctuation of the reflected image.